1. Field of the Invention
The present invention relates to minimizing errors in the components of the optical system of an extreme ultraviolet lithography (EUVL) system, and more specifically, it relates to a method and apparatus for performing figure corrections in multilayer-coated optics.
2. Description of Related Art
EUV lithographic optical systems require aspheric surfaces to be fabricated better than the most accurate mirror optics ever made. This requires improvements both in the polishing of optics and in the measurement of optics. This invention addresses both issues, by improving the precision to which EUV optics can be figured, primarily by making corrections to multilayer coated optics by etching or polishing the coating. The method also relieves the precision required in the measurement of the optics, since the etching has a much larger effect on visible wavelengths, used for measuring, than on the EUV wavelengths, used in use. Solutions that have been proposed by others for correcting mirrors have included depositing a film of material (such as Silicon) onto the multilayer and then polishing that film, without disturbing the multilayer. The figured film acts as a phase corrector, but it also causes absorption.
A thin-film phase corrector has been described by Mackowski et al. “VIRGO Mirrors: Wavefront Control,” Opt and Quant. Elec. 31, 507-514 (1999) (for the case of visible-light optics) and Yamamoto and Hatano, “EUV Reflection Phase Correction At A Multilayer Surface,” XEL-99 conference (for the case of EUV optics). In both these cases a thin film, acting as a refractive phase corrector, is deposited on the surface. In the latter case, this film is deposited on to the top of the multilayer coating.
See also U.S. Pat. No. 5,948,468, by Sweatt and Weed, titled “Method Of Correcting Imperfections On A Surface,” (Sep. 7, 1999), and U.S. patent application serial No. 10/086,922 by Taylor et al, titled “Correction Of Localized Shape Errors On Optical Surfaces By Altering The Localized Density Of Surface Or Near-Surface Layers,” incorporated herein by reference. In these methods, the substrate height errors are corrected directly, with the use of a thin film deposited on the substrate. In the case of Sweatt and Weed, the film is deposited through a specially designed mask so that its thickness varies across the substrate in such a way as to fill in the height errors of the substrate. In the case of Taylor et al. the correcting film is a uniform multilayer film. This film is chosen not for its reflective properties, but for its mechanical property of changing density upon heating. By applying heat locally and in a controlled way, the thickness of the film can be precisely modified to cancel out the substrate errors. In both of these methods the corrector film deposition and processing would be carried out prior to multilayer coating. In these cases, the thickness of film required is equal to the height error of the substrate, and there is no refractive factor.